The invention relates to equalizing arrangements for low temperature lines in general and more particualrly to such an arrangement which prevents undesired forces.
Low temperature lines having at least one rigid inner tube which contains at least one tubular compensating element for the equalization of length changes and is enclosed by an outer tube, the lines being used for the conduction of a cryogenic medium, are known. These and other low temperature lines are provided for the transport of gaseous or liquefied cryogenic media whose temperatures are considerably below the ambient temperature of these lines. In general, they contain at least one inner tube through which the cryogenic medium flows and which, therefore, has a temperature level in its operating state which at least approximates that of the cryogenic medium.
Low temperature cables having at least one dc or ac conductor cooled to low temperature and kept at low temperature by a cryogenic medium disposed in an inner tube are special types of low temperature lines. Where superconductive material is provided as the conductor material, the cryogenic medium is perferably helium. It is for this reason that the inner tube of such a cable is also called helium tube.
Such low temperature lines operate with relatively good thermal efficiency if additional measures are taken to limit heat exchange between the cryogenic medium in the inner tube and the outside temperature. In a special embodiment of such a low temperature line, therefore, the inner tube is concentrically enclosed by another tube. This other tube serves as a thermal shield and is also called a radiation shield. The radiation shield may expediently be kept, by another medium such as liquid nitrogen, at a temperature level higher than that of the inner tube. The radiation shield in turn is surrounded by an outer tube insuring the vacuum tightness of the entire low temperature line, this outer tube also acting as protection against mechanical damage to the inner tube and the radiation shield. Between the inner tube, in which are disposed, for example, electric conductors, and the outer tube there may, moreover, be disposed a large number of insulating foil layers to prevent heat transfer between the outer and inner tube. These insulating foil layers are also known as superinsulation. A stable positioning of the tubes enclosing each other is obtained by appropriate mechanical structures which allow simple assembly of these tubes and which keep the heat transfer between the tubes to a minimum.
One difficulty with such lines is that a rigid inner tube opposite a rigid outer tube changes its length when cooled to the operating temperature from the ambient temperature, or when it must be reheated from the operating temperature to the ambient temperature, such as in the case of a malfunction. For, all materials which can be used for the inner tubes to carry cryogenic media such as liquefied or gaseous nitrogen, hydrogen or helium, will shrink considerably when cooled from room temperature to the operating temperature. For example, at a temperature drop from 300 K to 4 K, this shrinkage amounts to 4.2.permill. for aluminum, 2.8.permill. for chrome-nickel steel and 3.2.permill. for copper. Even the shrinkage of special steel alloys, known by the name Invar, which shrink by only about 0.3.permill. for the temperature drop mentioned, cannot be neglected when long tubular lines are involved. In order to equalize length reductions of these lines it is generally necessary, therefore, to insert appropriate compensation elements such as sections of corrugated tubing in the inner tubes.
An appropriate embodiment of a low temperature line in which both the outer tube and the inner tube, in which conductor wires cooled to low temperature may be disposed, for example, are or rigid design, is described in the journal "Naturwissenschaften" 57 (1970), pages 414 to 422 in particular page 420, FIG. 7b.
The inner tube contains a compensating element in the form of a corrugated tube, by means of which shrinkage differences between the inner and outer tube, occurring when the inner tube is cooled to the operating temperature of the cryogenic medium flowing in it, can be equalized. Moreover, a radiation shield is disposed between the inner and the outer tube. This radiation shield is provided with cooling tubes in which another coolant may flow and which contain corresponding compensation elements for the equalization of shrinkage differences.
In order to keep the heat input into the cryogenic media carried by such low temperature lines as small as possible, a high vacuum is provided between the outer and the inner tube, and superinsulation and radiation shields are generally disposed between these tubes. In addition, it is necessary to fix the inner tube inside the outer tube so as to be as free of forces as possible in order to be able to design support or suspension devices of small cross-sectional area for the inner tube. It is then possible to keep the introduction of heat through these devices correspondingly low.
However, a study of the function of the cooling circuits for such low temperature lines during a cooling process, during operation or while their inner tubes are warming up will reveal that the known corrugated tube compensating elements only imperfectly meet the requirement that the inner tubes be held inside the outer tubes without stress. For example, in order to cool an inner tube continuously from room temperature to the intended operating temperature by means of a cooling gas such as helium, a considerable pressure such as of 10 to 15 bar is required because of the relatively low heat capacity of the gas. The tube diameter of inner tubes which may be intended to accommodate superconducting phase conductors of a superconducting cable are as large as 120 mm or even larger, for example. If a corrugated tube is now inserted in such an inner tube, it will be pushed apart with a corresponding, considerable force which may be as great as 1.1 to 1.7 tons. But the elastic force of a corrugated tube beyond its unstressed position is generally only 100 to 120 kg. Therefore, it is well below that sufficient to counteract the force acting on the corrugated tube. In order to not overstress the corrugated tube, it must, therefore generally be provided with a travel limiting device.
At the start of a cooling process of such a low temperature line whose inner tube is still warm, a corrugated tube inserted into the inner tube will therefore be expanded, due to the internal pressure, to its maximum length predetermined by the travel limiting device. The stretched inner tube is then too long by an amount equal to the travel of the corrugated tube. If, for example, the terminations of such a low temperature line are disposed perpendicular to the longitudinal direction of the line, the flanges at the terminations and at the required elbows are stressed in shear. If, on the other hand, the terminations are in the continuation of the straight inner tube, the corrugated tube will be compressed, but the terminations will be acted upon by the force mentioned.
Similar conditions prevail if a boiling cryogenic medium is immediately admitted to an inner tube of the low temperature line. The cryogenic medium then enters the inner tube, which is still at room temperature, at one end of the low temperature line. It is evaporated immediately and heated to room temperature after a short transition zone. In this process, a pressure of several bar is generated which exerts a force on the corrugated tube in the manner already described. Therefore, the corrugated tube is pushed apart while almost the entire length of the inner tube is still at room temperature. The same stresses will then occur at the terminations.
The forces occurring during the cooling processes described above dissipate only gradually with dropping inner tube temperature and disappear only when the operating temperature has been attained over the entire length of the inner tube. This means that the corrugated tube inserted in the inner tube is actually ineffective because it is constantly in its limit position determined by the travel limiting device. It behavior during a cooling process is approximately that of a smooth, rigid piece of tubing.